The aim of this work is to determine the DNA sequence organization of class I genes contained in the swine major histocompatibility complex (SLA). Both genomic lambda and cosmid libraries have been constructed from a homozygous SLAdd animal. From these, two non-allelic porcine class I SLA genes have been isolated and characterized. Both genes are expressed in mouse L cells, directing the synthesis of class i SLA molecules which carry common monomorphic determinants, but are serologically distinct. The corresponding DNA sequences have been determined. The organization of both of these gene is similar to that of other class I genes; consisting of 8 exons encoding a leader polypeptide, three extra cellular domains, a transmembrane and intracytoplasmic domain. The two genes are highly homologous in both exon and intron segments, with average homologies of 88 and 80%, respectively. Nucleotide changes in exon 2 are clustered, whereas those in the other exons are dispersed throughout. Comparison of the swine DNA sequences with class I genes from other species reveals a generally high conservation of exons 2, 3, 4 and 6. The data indicate an order of swine greater than human greater than rabbit greater than mouse in the relationship of class I genes.